This invention relates generally to a method of sputter deposition (sputtering) of thin film layers in a semiconductor process. More particularly, the present invention relates to a method of sputtering ferroelectric thin films such as PZT (lead zirconate titanate) and the like, as well as other metal oxide thin films using low frequency or pulsed DC power supplies to excite the sputtering target.
The sputtering of ferroelectric and metal oxide thin films is typically done using a high frequency radio frequency (RF) power supply to excite the sputtering target. A typical frequency for the power supply voltage is 13.56 MHz. A typical prior art RF sputtering configuration 10 is shown in FIG. 1. An RF power supply 12 generates an AC waveform, which is impedance matched via an RF matching network 14 to the characteristics of the PZT deposition target 16.
While the high frequency sputtering configuration of FIG. 1 is generally adequate for depositing ferroelectric and metal oxide thin films, there are several problems associated with this technique.
With high frequency RF power supplies, higher deposition pressures are typically required to enhance lead sticking efficiency, which is essential in forming a manufacturable and reliable ferroelectric thin film. Even though higher deposition pressures improve lead sticking efficiency, they do so at the expense of poorer cross-wafer film thickness uniformity due to reductions in plasma confinement by the magnetron sources.
The deposition rate using a high frequency power supply is also somewhat limited because the sputtering only occurs during half of the sinusoidal duty cycle.
Radio Frequency Interference (RFI) issues typically plague RF sputter process repeatability arid monitoring capabilities when a high frequency RF power supply is used. This is a constant problem with the RF supplies currently running on industry standard stutter deposition tools. Levels of RFI are picked up by electronic equipment within the sputtering tool itself, as well as in the surrounding lab or manufacturing areas. This can cause inaccurate readings on monitoring devices such as pressure readouts or the like, as well as occurrences of interference with test equipment in adjacent manufacturing areas.
At high RF frequencies, PZT constituents (lead and calcium) track each other with deposition pressure. The high frequency RF sputtering mode is therefore disadvantaged, because the relatively low peak power for a given average power setting causes the molecules sputtered from the PZT target to be relatively unfragmented.
The sticking efficiency of the lead and lack of fragmentation of the PZT and dopants from the sputtered target source are potentially responsible for nucleation defects that impact the quality oft he PZT crystal orientation. It is difficult to achieve the desired high peak power levels in the RF sputtering mode due to excessive target heating caused by a significant increase in average power.
What is desired, therefore, is a sputtering method for ferroelectric and metal oxide thin films that solves the problems of film thickness uniformity, low deposition rate, RF interference, film constituent tracking, defects impacting crystal orientation, and low wafer throughput found in the prior art RF sputtering method.
It is, therefore, a principal object of the present invention to provide a sputtering method that solves the RFI problem and other associated problems of the prior art RF high frequency sputtering method.
It is an advantage of the invention that the lead and calcium constituents in a deposited ferroelectric PZT thin film can be separately controlled due to the improvement in lead control over a narrower pressure range.
It is a further advantage of the invention that the pulsed DC sputtering method allows an additional ability to fine tune the lead and calcium constituents with pulse-width selection.
It is a further advantage of the invention t hat the Ti/Zr ratio in deposited ferroelectric PZT films becomes tunable with deposition power.
It is a further advantage of the invention that improvements in preferred PZT crystal orientation are obtained when compared to compositions generated using the prior art high frequency sputtering method, producing thin films with enhanced ferroelectric performance.
It is a further advantage of the invention that an increased range for lead contents from a given deposition target allows multi-layer depositions for enhanced PZT ferroelectric performance from a single sputtering target.
It is yet a further advantage of the invention that increased wafer throughput can be generated, reducing the cost of PZT processing.
According to the present invention a sputtering method is provided as an alternative to the prior art sputtering method using high frequency (13.56 MHz) RF power supplies for sputter deposition from nonconducting metal oxide, ceramic, and ferroelectric targets. Enhancements in deposition rate and composition control have been demonstrated using a pulsed DC sputtering method using a power supply in the frequency range of 100 to 250 KHz and a low frequency RF sputtering method using a power supply in the range of 200 to 500 KHz. The enhancement in composition control comes from an improvement in the sticking efficiencies of the volatile components in ferroelectric films. The low frequency and/or pulsed DC supplies provide lead content control for optimizing ferroelectric performance in pressure regimes that favor better cross-wafer composition and thickness uniformity in PVD (Physical Vapor Deposition) sputtering tools. Because of the enhanced composition control, the low frequency pulsed DC and RF supplies can easily match the compositions generated by existing prior art high frequency RF approaches, but with significantly higher deposition rates and more favorable sputter pressure regimes. In addition to the improvements in process capability, the pulsed DC and low frequency supplies do not exhibit the radio frequency interference issues which typically plague RF sputter process repeatability and monitoring capabilities.
The sputtering methods of the present invention can be used to obtain similar enhancements in other ferroelectric bodies as well. The application of low frequency pulsed DC in producing sputtered Aluminum Oxide (AI203) and Iridium Oxide (IrOX) films for applications in ferroelectric processing would have significant benefits such as improved deposition rates, improved film quality, and elimination of arcing from target surface poisoning.
The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention which proceeds with reference to the accompanying drawings.